Solid electrolytic capacitor

ABSTRACT

There is a solid electrolytic capacitor having a low ESR characteristic. In accordance with the solid electrolytic capacitor using an capacitor element wherein solid electrolyte is formed, the post-etching void factor of the anode foil used for the capacitor element is set at not more than 51% or the post-etching void factor of the cathode foil used for the capacitor element is set at not more than 44%, so that an electrode foil increases in conductivity and decreases in resistivity, to thereby obtain, coupled with the solid electrolyte of a low resistivity, a solid electrolytic capacitor having a lower ESR characteristic.

TECHNICAL FIELD

[0001] The present invention relates to a solid electrolytic capacitorusing a solid electrolyte as an electrolyte.

BACKGROUND ART

[0002] Electronic information devices have been digitalized in recentyears, and the increase in driving frequency of microprocessor units(MPU) as a core part of these electronic information devices has been inprogress, resulting in an increase in electric power consumption, andraising a remarkable problem of reliability caused by heat generation.As a countermeasure against this, attempts to reduce the driving voltagehave been made. As a circuit for supplying a highly accurate electricpower to the microprocessor, a DC-DC converter so called as a voltageregulator module (VRM) has widely been used. For an output-sidecapacitor, a large number of capacitors with low equivalent seriesresistances (ESR) are used for preventing any voltage drop. As acapacitor having this low ESR characteristic, a solid electrolyticcapacitor using a solid electrolyte as an electrolyte has been practicedin use and widely used as a capacitor suitable for these purposes.

[0003] The increase in driving frequency of the microprocessor has beenremarkable, however, with increasing the power consumption. In order torespond to that, the increase of the power supplied from the capacitorhas been requested for preventing any voltage drop. In other words, alarge power supply must be made in a short time, for which purpose theabove-mentioned solid electrolytic capacitor is needed to not only beincreased in capacity and decreased in size and voltage but also have anESR characteristic lower than ever.

[0004] The solid electrolytic capacitor will be then described. A valveaction metal foil such as aluminum, tantalum and niobium is subjected toan etching for increasing a surface-area thereof. Then, an etching isperformed on the anode foil on which an anode foil oxide film was formedand also on the valve action metal foil such as aluminum, tantalum andniobium, so that a cathode foil is formed. The anode foil and thecathode foil is laminated through a separator comprising kraft paper,Manila paper, glass separator, or nonwoven fabric made of syntheticfiber such as vinylon and polyester. After an anode plug and a cathodeplug are connected respectively to said anode foil and said cathode foilat optional positions, then the anode and cathode foils are rolled toform a capacitor element. A solid electrolyte is formed in thiscapacitor element, and the capacitor element is contained in a metalcase. Then, the opening of the metal case is sealed tightly by use of asealing resin comprising such resin as epoxy resin or by inserting asealing rubber in the opening for a closing process.

[0005] In accordance with the solid electrolytic capacitor constructedas described above, used as the electrolyte is a solid electrolytehaving a lower resistivity of not more than several tends Ω-cm comparedwith the conventional electrolytic solution having a resistivity of theorder of 100 Ω-cm. Accordingly, the solid electrolytic capacitor has agood ESR characteristic, as described above.

[0006] By the way, for the solid electrolyte, manganese dioxide with aresistivity of a few Ω-cm has been used conventionally. Later on, asolid electrolytic capacitor using polymers of TCNQ complex, polypyrroleand thiophene dielectric with resistivities of not more than 1 Ω-cm hasbeen put into practical use. With a further increase in drivingfrequency of MPU, there has been a request for a capacitor having asmaller size, a larger capacity and a lower ESR. According to thestudies by the inventors, such solid electrolytic capacitor has aninsufficient effect of reducing the ESR thereof, despite the lowresistivity of the electrolyte.

[0007] As described above, there is a limitation to the reduction of theESR of the capacitor solely by lowering the resistivity of the solidelectrolyte to thereby improve the solid electrolyte, and a furtherreduction of the ESR remains as a difficult problem.

[0008] The present invention was made to solve the above-describedproblem and provides a solid electrolytic capacitor which uses the solidelectrolyte of a low resistivity and achieves a further lower ESR.

DISCLOSURE OF THE INVENTION

[0009] Having made committed studies to solve the above problem, theinventors found out that if the void factor of the etched foil to beused for the electrode foil of the capacitor element is optimized, it ispossible to reduce the resistivity of the electrode foil of thecapacitor element and further reduce the ESR of the solid electrolyticcapacitor.

[0010] Namely, in accordance with the solid electrolytic capacitorwherein a solid electrolyte is formed in the capacitor element, anetched foil having a void factor of not more than 51% is used as theetched foil to be used for the anode foil of said capacitor element.This etched foil is subjected to conversion or anodic oxidation toobtain the anode foil of the present invention. By setting the voidfactor of the etched foil at 51% or less, the anode foil increases inconductivity and decreases in resistivity, to thereby achieve, coupledwith the solid electrolyte of a low resistivity, a solid electrolyticcapacitor having an unprecedentedly low ESR characteristic. If the voidfactor exceeds 51%, however, the anode foil increases in resistivity,and a desirable ESR characteristic cannot be obtained. Further, if thevoid factor is 20% or more, a high capacity is obtained and thus it ispreferable. The void factor of the etched foil means in this case avalue obtained by dividing the void volume of the etched foil by theapparent volume of the etched foil.

[0011] Further, if the foil thickness of the anode foil of theabove-described solid electrolytic capacitor is not less than 70 μm, theanode foil decreases in resistivity to reduce the ESR, while if the foilthickness of the anode foil is not more than 180 μm, downsizing of acapacitor is attainable, and thus it is preferable that the foilthickness is 70˜180 μm.

[0012] Still further, in accordance with the solid electrolyticcapacitor wherein the solid electrolyte is formed in the capacitorelement, an etched foil of a void factor of not more than 44% is used asthe etched foil to be used for the cathode foil of said capacitorelement. By setting the void factor of the etched foil at 44% or less,the cathode foil decreases in resistivity, to thereby achieve, coupledwith the organic semiconductor of a low resistivity, a solidelectrolytic capacitor having an unprecedentedly low ESR characteristic.If the void factor exceeds 44%, however, the cathode foil increases inresistivity, and a desirable ESR characteristic cannot be obtained.Further, if the void factor is 10% or more, a high capacity is obtained,and thus it is preferable.

[0013] Furthermore, if the foil thickness of the cathode foil of theabove-described solid electrolytic capacitor is in the rage of 50-180μm, the ESR further decreases and downsizing of a capacitor isattainable, and thus it is preferable.

[0014] To add mode, in the above-described solid electrolytic capacitorformed by rolling the electrode foils, the electrode foils of thecapacitor are long and the resistivity of the electrode foils themselvesis essentially large, for which reason the ESR-reducing effect of thewhole capacitor is extremely large.

[0015] If a TCNQ complex is used in this case as a solid electrolyte, itis possible to achieve a low ESR of 5˜8 mΩ in response to the recentrequests.

[0016] Moreover, if a polymer of thiophene dielectric represented by(Chem. 1) is used, it is possible to obtain an ESR the same as or higherthan the ESR when the TCNQ complex is used. In addition, the heatresistance of the capacitor improves, and thus it is preferable. Themost preferable among others is 3,4-ethylene-dioxy-thiophene having agood reactivity and good electric properties.

[0017] In this case, X is 0 or S; when X is O, A is alkylene orpolyoxyalkylene; and when at least either one of X is S, A is alkylene,polyoxyalkylene, substituted alkylene or substituted polyoxyalkylene,wherein the substituent is alkyl group, alkenyl group or alkoxy group.

[0018] Further, if manganese dioxide is used as the solid electrolyte,the heat resistance improves, and thus it is preferable.

[0019] THE BEST MODE FOR CARRYING OUT THE INVENTION

[0020] The embodiments of the present invention will be then describedspecifically. An aluminum foil is subjected to an alternate etching inan etching solution consisting of such solution as a hydrochloric acidsolution for surface roughening, and the void factor is set at not morethan 51%. Further, in order to form a dielectric film, the aluminum foilis subjected to a conversion in a conversion solution consisting of suchsolution as a phosphoric acid solution to form an anode foil. By settingthe void factor of an etched foil at not more than 51%, preferably notmore than 43%, and more preferably not more than 37%, the anode foilincreases in conductivity and decreases in resistivity, as a result ofwhich the ESR of the solid electrolytic capacitor is reduced. Asdescribed above, the present invention reduces the void factor forincreasing conductivity of the electrode foils so as to reduce the ESR,and thus the effect of the present invention will not decrease,independently of the conditions of the depth and the area ratio of anetching pit, for example, even on condition that the etching pits aredifferent in depth between the both sides of the etched foil.Furthermore, in order to ensure the maximum capacity-volumetricefficiency, if the void factor is not less than 20%, the etching areaincreases to obtain a high capacity, and thus it is preferable.

[0021] Moreover, if the foil thickness of the anode foil is not lessthan 70 μm, preferably not less than 90 μm, the anode foil decreases inresistivity to reduce the ESR, while if the foil thickness of the anodefoil is not more than 180 μm, preferably not more than 150 μm, thevolumetric efficiency of the capacitor improves, and thus downsizing ofa capacitor is attainable. Therefore, it is preferable that the foilthickness is 70-180 μm, more specifically 90-150 μm.

[0022] Further, similarly to the case of the anode foil, anotheraluminum foil is subjected to an alternate etching or a chemical etchingfor surface roughening, and the void factor is set at not more than 44%,preferably not more than 35%, and more preferably not more than 26%, tothereby form a cathode foil. By so doing, the cathode foil decreases inresistivity to reduce the, ESR of the solid electrolytic capacitor.Still further, in order to ensure the maximum capacity-volumetricefficiency, if the void factor is not less than 10%, the etching areaincreases to obtain a high capacity, and thus it is preferable.Moreover, if the foil thickness of the cathode foil is 50-180 ft in,preferably 70-150 t m, and more preferably 70100 m, the ESR furtherdecreases and downsizing of a capacitor becomes attainable, and thus itis preferable.

[0023] At this stage, in a roll-type solid electrolytic capacitorwherein an organic semiconductor having a low resistivity is used andthe void factor is reduced to increase conductivity of the electrodefoils, the resistance value per unit area of the electrode foils or theinter-end resistance value of the square electrode foils (hereinafter,the foil resistance rate), the foil width and the foil area areoptimized to further reduce the ESR of the capacitor.

[0024] In other words, by using an organic semiconductor having aresistivity of not more than 10 Ω·cm and the following electrode foil,it is possible to maximize the low resistivity characteristic of theorganic semiconductor. The foil resistance rate of the electrode foilsused for the present invention is 0.15˜0.6 mΩ, more preferably 0.17˜0.45mΩ. By using the electrode foils of this range, the electrode foilsdecrease in resistivity to reduce the ESR. With the foil resistance ratebelow this range, the ESR-reducing effect is small, while with the foilresistance rate above this range, the ESR-reducing rate decreases.

[0025] Further, the foil width is 3˜16 mm, preferably 5˜14 mm. With thefoil width below this range, even if the foil resistance rate isreduced, the contribution of resistivity of the electrolyte increases sothat the ESR will not decrease. If the foil width exceeds this range,the ESR-reducing rate decreases, and the length of the capacitor elementincreases, so that the impregnation characteristic of the organicsemiconductor decreases to reduce the electrolyte retention. As aresult, the ESR increases.

[0026] The foil area needs to be not smaller than 300 mm², preferablynot smaller than 500 mm². With the foil area below this range, the areasof the electrode foils and the electrolyte are small and the resistancerate will not decrease, as a result of which the ESR will not decrease.Further, the solid electrolytic capacitor according to the presentinvention uses a capacitor element formed by rolling the electrode foilslaminated through a separator, for which reason an electrode foil havinga long foil length can be used. Accordingly, a sufficient foil area isobtained, and the ESR decreases.

[0027] As described above, used for a cathode foil is an etched foil,and used for an anode foil is this etched foil, which is further turnedon electricity in a conversion solution consisting of such solution as aphosphoric acid solution for forming an oxide film on the surface ofthis etched foil. Therefore, such electrode foil comprises a non-etchingaluminum portion (hereinafter, a left-core) and an etching portion, andin case of an anode foil, the foil further comprises an oxide filmportion. By setting the thickness of this left-core at 50-170 μm, morepreferably 60˜150 μm, the foil resistance rate can be set at 0.15˜0.6mΩ, more preferably 0.17˜40.45 mΩ. If the thickness of the left-core istoo large, it becomes difficult to carry out the rolling operation.Therefore, it is preferable that the thickness of the left-core is notmore than 100 μm.

[0028] If the a TCNQ complex is used in this case as the organicsemiconductor, a low ESR of 4˜6 mΩ is achieved in response to the recentrequests.

[0029] Further, even if a conductive polymer is used as the organicsemiconductor, it is possible to obtain an ESR the same as or higherthan the ESR when the TCNQ complex is used. Furthermore, if a polymer ofthe thiophene dielectric represented by (Chem.1) is used as theconductive polymer, the heat resistance of the capacitor is improved,and thus it is preferable. The most preferable among others is3,4-ethylene-dioxy-thiophene having a good reactivity and good electricproperties.

[0030] For the surface-packaged solid electrolytic capacitor, which isrequired to be downsized, the width and the length of the electrode foilhave to be as small as possible. In the surface-packaged roll-type solidelectrolytic capacitor wherein a conductive polymer of a low resistivityis used and the void factor is reduced to increase conductivity of theelectrode foils, the ESR of the capacitor can be further reduced byoptimizing the foil resistance rate, the foil width and the foil area.

[0031] In other words, by using a conductive polymer with a resistivityof not more than 0.1 Ω·cm and the following electrode foil, it ispossible to maximize the low resistivity characteristic of theconductive polymer, thereby realizing a solid electrolytic capacitorhaving an unprecedentedly low ESR characteristic. The foil resistancerate of the electrode foils used for the present invention is 0.27˜0.7mΩ, more preferably 0.34˜0.55 mΩ. By using the electrode foils having aresistance rate of this range, the electrode foils decrease inresistivity to reduce the ESR. With the foil resistance rate below thisrange, the ESR-reducing effect is small, while with the foil resistancerate above this range, the ESR-reducing rate decreases.

[0032] Further, the foil width is 1.5˜4 mm, preferably 2˜3 mm. With thefoil width below this range, even if the foil resistance rate isreduced, the contribution of resistivity of the electrolyte increases sothat the ESR will not decrease. If the foil width exceeds this range,the ESR-reducing rate decreases, and the height of the capacitorincreases to exceed the tolerance limit of the surface-packagedcapacitor.

[0033] The foil area needs to be not smaller than 150 mm², preferablynot smaller than 200 mm². With the foil area below this range, the areasof the electrode foils and the electrolyte are small and the resistancerate will not decrease, as a result of which the ESR will not decrease.Further, the solid electrolytic capacitor according to the presentinvention uses a capacitor element formed by rolling the electrode foilslaminated through a separator, for which reason an electrode foil havinga long foil length can be used. Accordingly, a sufficient foil area isobtained, and the ESR decreases.

[0034] As described above, in order to set the foil resistance rate ofthe electrode foil at 0.27˜0.7 mΩ, and more preferably 0.34˜0.55 mΩ, itis possible to use an electrode foil having a left-core thickness of40˜100 μm, and more preferably 50˜80 mΩ.

[0035] Moreover, if a polymer of thiophene dielectric represented by(Chem. 1) is used as the conductive polymer, the ESR of the capacitor isreduced due to the low resistivity characteristic of not more than 0.1Ω·cm, and the heat resisting characteristic improves, which ispreferable. The most preferable among others is3,4-ethylene-dioxy-thiophene having a good reactivity and good electricproperties.

[0036] Further, if a conversion film of 0.1V˜10V, preferably 0.3˜5V, isformed on the cathode foil, the ESR decreases and theoperating-life-at-high-temperature improves, and thus it is preferable.

[0037] Furthermore, if a layer comprising a metallic compound or a metalof a low oxidizing characteristic such as titanium nitride and titaniumis formed on the surface of the cathode electrode, the electriccapacitance increases, and thus it is preferable. It is furtherpreferable if a conversion film is formed on the cathode electrode, andif the above-described layer comprising a metallic compound or a metalof a low oxidizing characteristic is formed on this conversion film.

[0038] The above-described anode foil and cathode foil are connectedrespectively with an anode-plug and a cathode-plug, and the anode foiland cathode foil laminated through a separator are rolled. Then, avoltage is applied to the laminated structure in a conversion solutionfor repairing the dielectric oxide fil m which was damaged during theprior processes. It is possible to obtain the effect of the presentinvention by using the above-described anode foil of the presentinvention and a conventional cathode foil or a conventional anode foiland the above-described cathode foil of the present invention. Themaximum effect can be obtained by using both of the anode foil and thecathode foil of the present invention.

[0039] Usable as a separator in this case are Manila paper, kraft paper,glass separator, nonwoven fabric made of synthetic fiber such as vinylonand polyester, and porous separator.

[0040] When a drawing means is connected with an electrode foil, if usedis the electrode foil of the present invention with an increasedconductivity, the contact resistance of the jointing of the electrodefoil and the electrode-drawing means decreases to further reduce the ESRof the solid electrolytic capacitor.

[0041] A case of using a TCNQ complex as the solid electrolyte will bethen described. The TCNQ complex is put in a cylindrical metal casecomprising aluminum, and the case is placed on a heated flatbed heaterfor melting and liquefying the TCNQ complex. Then, the preheatedabove-described capacitor element is impregnated there, and the metalcase is immersed in a cooling water for cooling and solidifying the TCNQcomplex. Further, epoxy resin is poured into the case for heating andsolidifying the content in the case in a high-temperature atmosphere.Thereafter, the content is subjected to heating and voltage applicationfor a subsequent aging process, to thereby form a solid electrolyticcapacitor.

[0042] In case that poly-(3,4-ethylene-dioxy-thiophene) (PEDT) or thepolymer of 3,4-ethylene-dioxy-thiophene (EDI) is used as the solidelectrolyte, the capacitor element is immersed in a solution prepared bymixing EDT, an oxidizing agent and a predetermined solvent for causingthe EDT polymerization reaction in the capacitor element, to therebyform a solid electrolyte layer comprising PEDT. Then, this capacitorelement is inserted in a metal case, and a sealing rubber is inserted inthe opening edge of the metal case for sealing the same by the closingprocess, to thereby complete a solid electrolytic capacitor.

[0043] It is possible to use EDT monomer as the above-described EDT. Itis also possible to use a monomer solution prepared by mixing EDT with avolatile solvent, wherein the volume ratio between EDT and the volatilesolvent is in the range of 1:0˜1:3. Usable as the above-describedvolatile solvent are hydrocarbons such as pentane, ethers such astetrahydrofuran, esters such as ethyl formate, ketones such as acetone,alcohols such as methanol, and nitrogen compounds such as acetonitrile.Preferable among them are methanol, ethanol and acetone. Usable as theoxidizing agent are ferric para-toluenesulfonate dissolved with butanoland a solution of periodic acid or iodic acid. The preferableconcentration of the oxidizing agent to the solvent is 40˜50 wt %. Ifthe concentration is below this range, the ESR increases, while if theconcentration exceeds this range, the electric capacitance decreases.

[0044] The compounding ratio between EDT and the oxidizing agent (thesolvent is not included) is preferably 1:0.9˜1:2.2, and more preferably1:1.3˜1:2.0 by weight ratio. If the ratio is outside this range, the ESRincreases. The conceivable reason for this is as follows. Namely, if theamount of the oxidizing agent to the monomer is too large, the relativeamount of the monomer to be impregnated decreases, so that the amount ofthe PEDT to be formed decreases to increase the ESR. On the other hand,if the amount of the oxidizing agent is too small, the oxidizing agentnecessary for polymerizing the monomer is insufficient, so that theamount of the PEDT to be formed decreases to increase the ESR.

[0045] As well as the above-described EDT, also applicable is apolymeric monomer. Usable as the polymeric monomer are aniline, pyrrole,furan and acetylene, and their derivatives, provided that they aresubjected to oxidative polymerization by a predetermined oxidizing agentto form a conductive polymer.

[0046] In case that manganese dioxide is used as the solid electrolyteand as the inorganic Electrolyte, the capacitor element is immersed in amanganese nitrate solution before heat-treating the same at atemperature of 300˜400°. Then, a voltage is applied to the capacitorelement again in a conversion solution for repair conversion of thedielectric oxide film which was damaged during the heat treatment. Aseries of the processes, for example, the immersion in the manganesenitrate solution, the heat treatment, and the repair conversion isrepeated a few times. Lastly, the capacitor element is subjected to aheat treatment at a temperature of 400˜500° C. for another repairconversion. The capacitor element wherein manganese dioxide was formedas the solid electrolyte as described above is then contained in anouter case. Then, epoxy resin is poured into the case for heating andsolidifying the content in the case before the case is sealed, tothereby form a solid electrolytic capacitor. As the above-describedinorganic electrolyte, such materials as lead dioxide can be used.

[0047] Specific examples of the solid electrolytic capacitor of thepresent invention will be then described below.

EXAMPLE 1

[0048] Examples using a TSNQ complex as the solid electrolyte will bedescribed. An aluminum foil is subjected to an alternate etching forsurface roughening and then to a conversion for forming a dielectricoxide film, to thereby form the anode foil of the present invention.Further, another aluminum foil is similarly subjected to an alternateetching for surface roughening, so that a conversion film is formed onthe surface thereof to form a cathode foil. The anode foil and thecathode foil are laminated through a separator comprising a Manilapaper, and an anode-plug and a cathode-plug are connected respectivelyto the anode foil and the cathode foil at optional positions, before thelamination structure is rolled. Then, a voltage is applied to thelaminated structure in a conversion solution for repair conversion ofthe dielectric oxide film which was damaged by the rolling operation.

[0049] On the other hand, the TCNQ complex is put in a cylindrical metalcase comprising aluminum, and the case is then placed on a flatbedheater heated at approximately 280° C. for melting and liquefying theTCNQ complex. Then, the above-described capacitor element, which waspreheated approximately 300° C., is impregnated there, and the metalcase is promptly immersed in a cooling water for cooling and solidifyingthe TCNQ complex. Further, a necessary amount of epoxy resin is pouredinto the case for heating and solidifying the content in the case in ahigh-temperature atmosphere. Thereafter, the rated voltage is appliedbetween the terminals at a temperature of 125° C. for one hour for asubsequent aging process, to thereby form a solid electrolytic capacitorof 10φ×10 L.

EXAMPLE 1-1

[0050] For a solid electrolytic capacitor formed as described above, afoil having a thickness of 100 μm and a post-etching void factor of 26%was used as an anode foil and a foil having a thickness of 85 μm and apost-etching void factor of 19% was used as a cathode foil, so as to usethe solid electrolytic capacitor as the example 1-1.

EXAMPLE 1-2

[0051] For another solid electrolytic capacitor formed as describedabove, a foil having a thickness of 100 μm and a post-etching voidfactor of 46% was used as an anode foil and a foil having a thickness of50 μm and a post-etching void factor of 39% was used as a cathode foil,so as to use the solid electrolytic capacitor as the example 1-2.

EXAMPLE 1-3

[0052] Still another solid electrolytic capacitor was formed by the samemeasure used for the example 1-2, except that a foil having a thicknessof 100 μm and a post-etching void factor of 33% was used as an anodefoil, so as to use the solid electrolytic capacitor as the example 1-3.

EXAMPLE 1-4

[0053] For yet another solid electrolytic capacitor formed as describedabove, a foil having a thickness of 100 μm and a post-etching voidfactor of 52% was used as an anode foil and a foil having a thickness of75 μm and a post-etching void factor of 22% was used as a cathode foil,so as to use the solid electrolytic capacitor as the example 1-4.

CONVENTIONAL EXAMPLE 1

[0054] Further, for another solid electrolytic capacitor formed asdescribed above, a foil having a thickness of 100 μm and a post-etchingvoid factor of 52% was used as an anode foil and a foil having athickness of 50 μm and a post-etching void factor of 39% was used as acathode foil, so as to use the solid electrolytic capacitor as theconventional example.

[0055] (Table 1) represents the comparisons between the foil resistancesof the anode foils and the ESRs of the solid electrolytic capacitors ofthe examples 1-1˜1-4 and the conventional example. TABLE 1 Anode foilCathode foil Void Foil Foil Void Foil factor thickness resistance factorthickness ESR (%) (μm) (mΩ/m) (%) (μm) (mΩ) Example 1-1 26 100 75 19 855.8 Example 1-2 46 100 153 39 50 8.5 Example 1-3 33 100 91 39 50 7.8Example 1-4 52 100 231 22 75 7.7 Conventional 52 100 231 39 50 9.2example 1

[0056] As can be seen from (Table 1), the ESR is reduced in the examples1-2 and 1-3 using the anode foil of the present invention and in theexample 1-4 using the cathode foil of the present invention, comparedwith the conventional example. Further, in the example 1-1 wherein ananode foil having a void factor of 26% and a cathode foil having a voidfactor of 19% are used, the ESR is reduced to 5.8 mΩ, achieving anunprecedentedly low ESR. Still further, the ESR is reduced more in theexample 1-3 wherein an anode foil having a void factor of 33% is usedthan in the example 1-2 wherein an anode foil having a void factor of46% is used.

[0057] Furthermore, as the void factor is reduced from 52% of theconventional example to 26˜46% of the examples, the foil resistance isreduced from 231 mΩ/m to 75-153 mΩ/m. This reveals that an electrodefoil increases in conductivity and decreases in resistivity.

[0058] As a comparative example, an electrolytic element was formed in asimilar way to the example 1-1 and the electrolytic element wasimpregnated with an electrolytic solution having a low resistivitycharacteristic, to thereby form an electrolytic capacitor. The usedelectrolytic solution comprises 75 parts of γ-butyrolactone and 25 partsof ethyl phthalate-dimethyl-imidazolium. The obtained ESR is a highvalue of 52 mΩ, which reveals that the effect of the present inventioncannot be obtained even with the use of the electrode foils of thepresent invention, if the solid electrolyte having a low resistivitycharacteristic is not used as the electrolyte.

EXAMPLE 2

[0059] An example using PEDT as the solid electrolyte will be thendescribed. A nonwoven fabric made of vinylon fiber was used for aseparator. For the cathode foil, a layer comprising titanium nitride wasformed on a conversion film. Except for this, the same steps as those ofthe example 1 were taken to form a capacitor element, and then a repairconversion was carried out. Then, a solid electrolyte was formed asfollows. A compound solution was prepared in a cup container by pouringtherein EDT and a butanol solution containing 45% of ferricpara-toluenesulfonate so that the weight ratio of the two become 1:0.8.Subsequently, the capacitor element was immersed in the above-describedsolution for ten seconds. The capacitor element was then heated at atemperature of 120° C. for one hour for causing the PEDT polymerizationreaction in the capacitor element, to thereby form a solid electrolytelayer. Then, this capacitor element was inserted in a cylinder-shapedbottom-closed aluminum case, and the opening of the case was sealed withrubber by the closing process for a subsequent aging process, to therebyform a solid electrolytic capacitor of 10φ×10 L. Solid electrolyticcapacitors using the same electrode foil of the example 1 were formed tobe used as the examples 3-1˜3-4 and the conventional example 3. (Table2) represents the comparisons between the ESRs of these solidelectrolytic capacitors. TABLE 2 Anode foil Cathode foil Foil Foil Voidfactor thickness Void factor thickness ESR (%) (μm) (%) (μm) (mΩ)Example 2-1 26 100 19 85 5.0 Example 2-2 46 100 39 50 6.8 Example 2-3 33100 39 50 7.2 Example 2-4 52 100 22 75 6.8 Conventional 52 100 39 50 8.1example 2

[0060] As can be seen from (Table 2), the example 2 shows the sameresult as that of the example 1, namely, the example 2 reveals theeffect of the present invention.

EXAMPLE 3

[0061] Subsequently formed is a surface-packaged chip-type solidelectrolytic capacitor of a capacitor size of 6.3φ×6 L. The solidelectrolytic capacitors formed in the same way as the example 1 wereused for the examples 3-1, 3-2 and the conventional example 3-1. Thesolid electrolytic capacitors formed in the same way as the example 2were used for the examples 3-3-3-6 and the conventional example 3-2.(Table 3) represents the void factor and the foil thickness of each ofthe used electrode foils, and the ESR of each of those solidelectrolytic capacitors. Anode foil Cathode foil Void Foil Void Foilfactor thickness factor thickness Solid ESR (%) (μm) (%) (μm)electrolyte (mΩ) Example 3-1 26 100 19 85 TCNQ 17.9 Example 3-2 33 10039 50 TCNQ 22.2 Example 3-3 26 100 19 85 PEDT 14.3 Example 3-4 46 100 3950 PEDT 17.4 Example 3-5 33 100 39 50 PEDT 18.3 Example 3-6 52 100 22 75PEDT 17.4 Conventional 52 100 39 50 TCNQ 23.7 example 3-1 Conventional52 100 39 50 PEDT 19.7 example 3-2

[0062] As can be seen from (Table 3), the chip-type solid electrolyticcapacitor of the example 3 shows the similar result to those of theexamples 1 and 2. Further, in the examples 3-1 and 3-3 wherein an anodefoil having a void factor of 26% and a cathode foil having a void factorof 19% are used, the ESR is reduced by 5.8 Ωm and 5.4 Ωm respectivelyfrom the conventional examples 3-1 and 3-2. Namely, the effect of thepresent invention is great. Still further, in the example 3-3 whereinPEDT is used as the solid electrolyte, the ESR is reduced by 3.6 mΩ fromthe example 3-1 wherein the TCNQ complex is used. The example 3-3, achip-type solid electrolytic capacitor, therefore achieves anunprecedentedly low ESR characteristic. Furthermore, a lead-free reflowtest was carried out at the peak temperature of 250° C. for the solidelectrolytic capacitors of the examples 3-3˜3-6 wherein PEDT is used asthe solid electrolyte. It was then found that there is no variation inelectric capacitance and ESR and that the lead-free reflow is applicableto those solid electrolytic capacitors.

EXAMPLE 4

[0063] An example using manganese dioxide as the solid electrolyte willbe then described, The capacitor element is immersed in a 40% solutionof manganese nitrate before heat-treating the same at a temperature of350° C. Then, a voltage is applied to the capacitor element again in aconversion solution for repair conversion of the dielectric oxide filmwhich was damaged during the heat treatment. This set process of theimmersion in the manganese nitrate solution, the heat treatment, and therepair conversion is repeated a few times. Lastly, the capacitor elementis subjected to a heat treatment at a temperature of 450° C. for anotherrepair conversion. This capacitor element wherein manganese dioxide wasformed as the solid electrolyte as described above is then contained inan outer case. Then, epoxy resin is poured into the case for heating andsolidifying the content in the case before the case is sealed, tothereby form a solid electrolytic capacitor of 10φ×10 L. Solidelectrolytic capacitors using the same electrode foil of the example 1were formed to be used as the examples 4-1, 4-2 and the conventionalexample 4. (Table 4) represents the void factor and the foil thicknessof each of the used electrode foils, and the ESR of each of those solidelectrolytic capacitors. TABLE 4 Anode foil Cathode foil Void Foil VoidFoil factor thickness factor thickness ESR (%) (μm) (%) (μm) (mΩ)Example 4-1 26 100 19 85 14.6 Example 4-2 33 100 39 50 17.7 Conventional52 100 39 50 20.1 example 2

[0064] As can be seen from (Table 4), the example 4 shows the similarresult to that of the example 1, revealing the effect of the presentinvention.

[0065] The present invention is not limited to the above-describedexamples, but the conditions for manufacturing solid electrolyticcapacitor such as the etching condition and the conversion condition canbe selected accordingly. Further, the solid electrolytic capacitor isnot limited to the roll-type, and thus such types as lamination is alsoapplicable.

[0066] To be described next is an example of the roll-type solidelectrolytic capacitor wherein the organic semiconductor is used and thefoil resistance rate, the foil width and the foil area are specified.

EXAMPLE 1

[0067] An example using the TCNQ complex as the organic semiconductorwill be then described. An aluminum foil is subjected to an alternateetching for surface roughening and then a conversion for forming adielectric oxide film, so that the abode foil of the present inventionis formed. Another aluminum foil is similarly subjected to an alternateetching for surface roughening, so that a conversion film is formed onthe surface thereof to form a cathode foil. The anode foil and thecathode foil are laminated through a separator comprising a Manilapaper, and an anode-plug and a cathode-plug are connected respectivelyto the anode foil and the cathode foil at optional positions, so thatthe lamination structure is rolled. Then, a voltage is applied to thelaminated structure in a conversion solution for repair conversion ofthe dielectric oxide film which was damaged by the rolling operation.

[0068] On the other hand, the TCNQ complex is put in a cylindrical metalcase comprising aluminum, and the case is then placed on a flatbedheater heated at approximately 280° C. for melting and liquefying theTCNQ complex. Then, the above-describcd capacitor element, which waspreheated approximately 300° C., is impregnated there, and the metalcase is promptly immersed in a cooling water for cooling and solidifyingthe TCNQ complex. Further, a necessary amount of epoxy resin is pouredinto the case for heating and solidifying the content in the case in ahigh-temperature atmosphere. Thereafter, the rated voltage is appliedbetween the terminals at a temperature of 125° C. for one hour for asubsequent aging process, to thereby form a solid electrolyticcapacitor.

[0069] (Table 5) represents the foil resistance rate, the foil width,the foil area and the left-core thickness of the anode foil and thecathode foil used in the examples 1-1˜1-3 and the comparative examples1-1˜1-4 of the solid electrolytic capacitor formed as described above,as well as the ESR of the solid electrolytic capacitor of each example.For the examples 1-1˜1-3, the foil thickness was 110 μm, 115 μm and 140μm, respectively. TABLE 5 Foil resistance Foil Foil Left-core rate widtharea thickness ESR (mΩ) (mm) (mm²) (μm) (mΩ) Example 1-1 0.40 6 1000 655.8 Example 1-2 0.41 11 1850 64 3.9 Example 1-3 0.25 6 800 102 5.7Comparative 0.89 6 1950 32 8.1 example 1-1 Comparative 0.22 2 2100 1199.8 example 1-2 Comparative 0.40 20 3400 65 7.1 example 1-3 Comparative0.17 15 250 145 11.2 example 1-4

[0070] As can be seen from (Table 5), the solid electrolytic capacitorsof the examples 1-1˜1-3 of the present invention have a low ESR value of6 mΩ, revealing the effect of the present invention. By contrast, in thecomparative example 1-1 wherein the foil resistance rate is not lessthan 0.6 mΩ, the ESR becomes a high value of 8.1 mΩ, even though theexample has the foil area of 1950 mm², which is larger than the foilareas of the examples 1-1˜1-3. Further, in the comparative example 1-2with the foil width of not more than 3 mm, the foil resistance rate is0.22 mΩ, which is lower than the foil resistance rates of the examples1-1˜1-3. Further, the comparative example 1-2 has a high ESR value of9.8 mΩ, even though the foil area of the example is 2100 mm², which islarger than the foil areas of the examples 1-1˜1-3. The comparativeexample 1-3 with a foil width of over 6 mm also has a high ESR value of7.1 mΩ. Still further, the comparative example 1-4 with a foil area ofnot larger than 300 mm² has a high ESR value of 11.2 mΩ, despite a smallfoil resistance rate of 0.17 mΩ, which is lower than the foil resistancerates of examples 1-1-1-3.

[0071] Furthermore, using the capacitor element formed in a similar wayto the examples 1-1, an electrolytic capacitor immersed with anelectrolytic solution with a low resistivity characteristic was formedas the comparative example 1-5, and an electrolytic capacitor withmanganese dioxide was formed as the comparative example 1-6. Theelectrolytic solution used for the comparative example 1-5 comprises 75parts of γ-butyrolactone and 25 parts of ethylphthalate-dimethyl-imidazolium. The obtained ESRs are respectively highvalues of 52 mΩ and 17 mΩ, which reveals that the effect of the presentinvention cannot be obtained even with the use of the electrode foils ofthe present invention, if an organic semiconductor having a lowresistivity characteristic is not used.

EXAMPLE 2

[0072] An example using PEDT as the organic semiconductor will be thendescribed. A nonwoven fabric made of vinylon fiber is used for aseparator. For the cathode foil, a layer comprising titanium nitride isformed on a conversion film. Except for this, the same steps as those ofthe example 1 were taken to form a capacitor element, and then a repairconversion was carried out. Then, an organic semiconductor was formed asfollows. A compound solution was prepared in a cup container by pouringtherein EDT and a butanol solution containing 45% ofpara-toluenesulfonic acid ferric so that the weight ratio of the twobecome 1:0.8. Subsequently, the capacitor element was immersed in theabove-described solution for ten seconds. The capacitor element was thenheated at a temperature of 120° C. for one hour for causing the PEDTpolymerization reaction in the capacitor element, to thereby form anorganic semiconductor layer. Then, this capacitor element was insertedin a cylinder-shaped bottom-closed aluminum case, and the opening of thecase was sealed with rubber by the closing process for a subsequentaging process, to thereby form a solid electrolytic capacitor. (Table 6)represents the foil resistance rate, the foil width, the foil area andthe left-core thickness of the electrode foil used in each of theexamples and the comparative examples, as well as the ESR of the solidelectrolytic capacitor of each example. For the examples 2-1˜2-3, thefoil thickness was 115 μm, 110 μm and 135 μm, respectively. TABLE 6 Foilresistance Foil Foil Left-core rate width area thickness ESR (mΩ/m²)(mm) (mm²) (μm) (mΩ) Example 2-1 0.41 6 1050 63 5.1 Example 2-2 0.40 111800 65 3.5 Example 2-3 0.24 6 850 105 4.9 Comparative 0.80 6 1450 307.2 example 2-1 Comparative 0.21 2 2150 121 8.6 example 2-2 Comparative0.40 20 3350 64 6.5 example 2-3 Comparative 0.17 15 240 150 9.0 example2-4

[0073] As can be seen from (Table 6), the example 2 shows the similarresult to that of the example 1, revealing the effect of the presentinvention.

[0074] To be described next is an example of the roll-type solidelectrolytic capacitor for surface-packaging wherein a conductivepolymer is used and the foil resistance rate, the foil width and thefoil area are specified.

[0075] An example using PEDT as a conductive polymer will be thendescribed. A nonwoven fabric made of vinylon fiber is used for aseparator. For the cathode foil, a layer comprising titanium nitride isformed on a conversion film. Except for this, the same steps as those ofthe example 1 were taken to form a capacitor element, and then a repairconversion was carried out. Then, a solid electrolyte was formed asfollows. A compound solution was prepared in a cup container by pouringtherein EDT and a butanol solution containing 45% ofpara-toluenesulfonic acid ferric so that the weight ratio of the twobecome 1:0.8. Subsequently, the capacitor element was immersed in theabove-described solution for ten seconds. The capacitor element was thenheated at a temperature of 120° C. for one hour for causing the PEDTpolymerization reaction in the capacitor element, to thereby form asolid electrolyte layer. Then, this capacitor element was inserted in acylinder-shaped bottom-closed aluminum case, and the opening of the casewas sealed with rubber by the closing process for a subsequent agingprocess, to thereby form a solid electrolytic capacitor. (Table 7)represents the foil resistance rate, the foil width, the foil area andthe left-core thickness of the anode foil and the cathode foil used ineach of the example 1-3 and the comparative example 14, as well as theESR of the solid electrolytic capacitor of each example. For theexamples 1-3, the foil thickness was 95 μm, 90 μm and 120 μm,respectively. TABLE 7 Foil resistance Foil Foil Left-core rate widtharea thickness ESR (mΩ) (mm) (mm²) (μm) (mΩ) Example 1 0.55 2.2 240 5013 Example 2 0.56 3 320 50 10 Example 3 0.34 2.2 170 80 14 Comparativeexample 1 0.91 2.2 335 31 17 Comparative example 2 0.31 1 360 85 20Comparative example 3 0.32 2.2 110 84 19

[0076] As can be seen from (able 7), the solid electrolytic capacitorsof the examples 1˜3 of the present invention have a low ESR value of notmore than 15 mΩ, revealing the effect of the present invention. Bycontrast, in the comparative example 1 wherein the foil resistance rateis not less than 0.7 mΩ, the ESR becomes a high value of 17 mΩ, eventhough the example has the foil area of 335 mm², which is larger thanthe foil areas of the examples 1˜3. Further, in the comparative example2 with the foil width of not more than 1.5 mm, the ESR is as high as 20mΩ, even though the foil resistance rate of the example is 0.31 mΩ,which is smaller than the foil resistance rates of the examples 1˜3, andalso the foil area of the example is 360 mm², which is larger than thefoil areas of the examples 1˜3. Still further, in the comparativeexample 3 with the foil area of not larger than 150 mm², the ESR becomesa large value of 19 mΩ, despite a low foil resistance rate of 0.32 mΩ,which is smaller than the foil resistance rates of the examples 1-3.

[0077] Furthermore, for the comparative examples 4 and 5, capacitorelements were formed in a similar way to the example 1. For both ofthese electrolytic capacitors, a TCNQ complex and manganese dioxide wereformed respectively as the solid electrolyte. The obtained ESRs arerespectively high values of 17 mΩ and 61 mΩ, which reveals that theeffect of the present invention cannot be obtained even with the use ofthe electrode foils of the present invention, if a conductive polymerhaving a low resistivity characteristic is not used as the electrolyte.

[0078] As described above, in accordance with the present invention, itis possible to achieve a solid electrolytic capacitor which uses a solidelectrolyte as the electrolyte and has an unprecedentedly low ESRcharacteristic, if the post-etching void factor of the anode foil usedfor the capacitor element is made not higher than 51%, and if thepost-etching void factor of the cathode foil used for the capacitorelement is made not higher than 44%.

1. A solid electrolytic capacitor comprising a capacitor element with asolid electrolyte, wherein said capacitor element includes an anode foilcomprising an etched foil having a void factor of not more than 51%. 2.A solid electrolytic capacitor comprising a capacitor element with asolid electrolyte, wherein said capacitor element includes a cathodefoil comprising an etched foil having a void factor of not more than44%.
 3. The solid electrolytic capacitor as claimed in claim 1, whereinsaid void factor is not less than 20%.
 4. The solid electrolyticcapacitor as claimed in claim 2, wherein said void factor is not lessthan 10%.
 5. The solid electrolytic capacitor as claimed in claim 1,wherein a thickness of said anode foil is 70˜180 μm.
 6. The solidelectrolytic capacitor as claimed in claim 2, wherein a thickness ofsaid cathode foil is 50˜180 μm.
 7. The solid electrolytic capacitor asclaimed in any one claims 1-6, wherein said capacitor element comprisesa roll of laminations a separator, and said anode and cathode foilsconnected with plugs at optional portions.
 8. The solid electrolyticcapacitor as claimed in claims 1˜7, wherein a TCNQ complex is used assaid solid electrolyte.
 9. The solid electrolytic capacitor as claimedin claims 1˜7, wherein a polymer of thiophene dielectric is used as saidsolid electrolyte.
 10. The solid electrolytic capacitor as claimed inclaim 9, wherein thiophene dielectric comprises3,4-ethylene-dioxy-thiophene.
 11. The solid electrolytic capacitor asclaimed in any one of claims 1-7, wherein manganese dioxide is used assaid solid electrolyte.